The present invention relates to new coatings for polyolefinic films, particularly but not exclusively, for polypropylene films and further relates to films and the like covered by said coatings, especially for rendering said films thermosealable by conventional packaging and sealing machines.
Polyolefinic films in general, and isotactic polypropylene films in particular, have experienced in the recent years a notable development especially in the field of sealed bag packaging and in general in the automatized packaging field.
The present films have been used in particular as replacement of the regenerated cellulose films, generally known as "Cellophane" films, as alternates but also often as substitutes to thin polyvinylchloride films with respect to which they offer numerous advantages.
Cellophane, as is well known, has been up to few years ago one of the most widely used materials in the field of flexible packaging for the food industry.
Often, in order to improve cellophane performance, it is necessary to treat the film with thermoplastic resins which serve to improve the sealability of two or more films with each other in a packaging machine. But, above all, said coatings served to form an effective barrier for gases and water vapor to which all cellulose derivatives are particularly sensitive. The above-described treatment is easily effected utilizing well knows systems of coating or the like, using for example coatings of the following compositions:
"Saran"--PVDC (dispersion or solution) and its copolymers PA0 PVC (dispersion or solution) and its copolymers PA0 Nitrocellulose (with various resins)
The application of the coatings described does not present any particular difficulties due to the very good thermal properties of the regenerated cellulose.
Plastic films, in general, and in particular mono- or bi-oriented (biaxially drawn) polypropylene films, even though possessing better physico-mechanical characteristics than cellophane, are very sensitive to temperatures higher than 110.degree. C. and are less stable thermally than cellophane.
Consequently they are more difficult to heat-seal, requiring special apparatus.
Therefore, the coating of said polypropylene films must, above all, eliminate their thermosealability deficiencies and thus permit the sealing thereof at temperatures lower than those at which the polypropylene may undergo the adverse effects of rippling, tightening, melting, degradations and the like.
To achieve this result, the polypropylene film (hereinafter referred to as "PP films") are coated using procedures similar to those used for cellophane but modified and improved to account for the different behavior of the material being treated. Among the materials most widely used for the coating of said films are vinyl resins and their copolymers, vinylicid resins and their copolymers, acrylic resins and their copolymers or mixtures of the above resins in varying proportions. The above resins are applied onto the PP film either as solutions in a suitable solvent or as emulsions.
The major difficulty in coating the PP base film consists in obtaining a good adhesion of the coating to the base film. In fact, a routing coating operation could result in very low sealability values due to the delamination of the coating. In order to increase the mechanical resistance of the seals, there are certain treatments to which the film is subjected to before the coating operation. The precoating treatments known at this time are: electric discharge treatment (corona effect), flame treatment, treatment with oxidizing solutions (impractical because of processing difficulties) and, finally treatment with a suitable primer, preferably effected in conjunction with the above-mentioned surface treatments.
The flame or corona treatments are easily performed and have a definite effect in improving the wettability properties of the base film. Among the primers, the better and most practical, and consequently the one most often used, is a water-soluble polyimine compound, used for some time in the paper industry and in the manufacturing industry (in the production of flexible plastic film articles consisting of combinations of plastic film with aluminum, paper and cellophane).
The technique of the above-mentioned technologies have already been used in the field of isotactic polyethylene films and there exist, in this regard, some patents to various companies in this field. Among these, an Italian patent to Montecatini discloses a process (Italian Pat. No. 717,612, filed on Mar. 16, 1964) for obtaining coated films (called "Moplefan") consisting substantially of a base film of polypropylene high in isotatic content. Said base film is subjected to a flame treatment and to a corona discharge treatment as above described. After said treatment, a polyimine based primer is applied on the base film. The treated film is then covered with a coating consisting of a mixture of, substantially, three basic components. In the patent there is claimed the effect of the oxiranic group contained in the epoxy resin in improving the adhesion of the coating to the support base. There are also known other commercial products such as "Prophan" having thermosealability characteristics similar to those of Moplefan.
There also exist alternate processes which utilize different technologies such as coupling and lamination of thin film, which also require the use of primers, adhesives and/or hot melts for the coupling of said films with a previously extruded film or with a molten layer. Also in this case, the effectiveness of polyimine as an adhesion promoter is known.
It is also important that the product, in addition to the thermosealability characteristics, must also have other characteristics such as: good optical transparency, no tackiness above 45.degree./60.degree. C., stability to aging, cohesion of the sealed film immediately after the opening of the sealing bars, easy separation from the sealing bars, minimum value of friction coefficient, low solvent retention, resistance to greases and fats, good printability, impermeability to gases, etc. Furthermore, in applications in the food packaging field, said films must comply with the existing norms in the field. In view of these facts, in the resin compositions there exist, for each type of base formulation, modifiers and additives which must be dosed and chosen as to optimize the distribution of values of the characteristics required for each specific application.